1. Field of the Invention
The present invention relates to a combustion monitoring system in general, and in particular to a system for monitoring conditions in the combustion system of a gas turbine.
2. Brief Discussion of the Related Art
Many industrial processes such as power generation, metal smelting and processing, waste incineration and vitrification, glass melting, crude oil refining, petrochemical production, and the like use burners as the primary or as an auxiliary source of energy. These burners have one or more inlets for hydrocarbon based fossil fuels such as, but not limited to, natural gas, liquefied petroleum gas, liquid hydrocarbon-based fuel, and the like, which are combusted to produce heat. The fuels are burned in a combustion chamber where the energy that is released by the combustion is transferred in the form of heat for the required purpose. The combustion requires an oxidant, such as air, oxygen-enriched air, or oxygen. In most cases, the oxidant is preheated in order to provide for more efficient combustion.
Precise monitoring and control of the combustion process is very important for the efficient and safe operation of industrial processes. For example, it is well known that burning a fuel with excess air as the oxidant yields higher nitrogen oxides (NOx) emissions, especially when the air is preheated. On the other hand, incomplete combustion of a fuel generates carbon monoxide (CO). Both NOx and CO are dangerous pollutants, and governmental environmental authorities regulate the emission of both gases.
Stringent environmental emission regulations have motivated changes in the design and operation of combustion processes, in particular gas combustion systems. Many developers of gas combustion systems, such as stationary gas turbines, use some form of lean-premix combustion (LPM). In LPM systems, fuel is mixed with air upstream of the combustion zone at deliberately fuel-lean conditions. A significant reduction of thermal NOx formation is achieved using LPM system. Research activities by both U.S. Government laboratories and the private sector have been conducted, with specific goals for NOx emissions of less that 10 ppm. To meet the target NOx levels, modem premix turbine combustors must operate with a finely controlled fuel/air ratio, near the lean extinction limit. In practice, changes in flow splits caused by manufacturing tolerances or engine wear can compromise emissions performance. Furthermore, unexpected changes in fuel composition, or momentary changes in fuel delivery can lead to problems with flame anchoring.
Serious engine damage can result when premixed flames flashback, yet there are currently no methods to sense when flashback may be incipient. Related problems can arise from autoignition, where fuel begins to burn in the premixer without any flashback. Because of the presence of heavy hydrocarbons or pipeline cleaning solvents in natural gas, the operating margin for autoignition may be compromised in high-pressure gas turbines. Likewise, operation near lean-blowoff is desired to reduce NOx emissions, but this complicates the change to different fuels, because the flame anchoring will be different on different fuels near lean-blowout.
Due to these issues, there is a growing need to both measure and control the behavior of flames and, in turn, the combustion process in gas turbine combustors. The measurement of combustion parameters when coupled with a combustion control strategy presents numerous unique issues due to the extreme process conditions under which the combustion process occurs.
Numerous systems are available for the measurement of flames in burners, and in particular gas turbines. For example, commercially available UV flame detectors can be used to monitor the status (flame on or off) of a flame. Alternatively, a photocell may be used as the detector. At least one element of the photocell is coated with a sulfide compound, such as cadmium-sulfide or lead-sulfide, so as to be sensitive to the particular wavelengths of light emitted by a flame occurring during a flashback condition. For instance, the electrical resistance of cadmium-sulfide decrease directly with increasing intensity of light, and like lead-sulfide, will function as a variable resistor. However, when used to detect the presence of a flame, a cadmium-sulfide photocell is useful only for sensing that portion of the flame occurring in the visible light wavelengths. Further, these types of flame monitoring device do not provide information on the combustion product mixture. It may be difficult to determine whether the burner is operated under fuel rich, fuel lean, or stoichiometric (exact amounts of fuel and oxidant to obtain complete combustion of the fuel, equivalence ratio equal to 1). Further, flame detectors based on the measurement of selected wavelengths of the electromagnetic spectrum are typically self contained devices that are not always integrated in the burner design.
Endoscopes may also be used within industry to visually inspect flames, and their interaction between the furnace load. They are generally complicated and expensive pieces of equipment that require careful maintenance. To be introduced into very high temperature furnaces or burners, they require external cooling and flushing means: high-pressure compressed air and water are the most common cooling fluids. When compressed air is used, uncontrolled amounts of air are introduced in the furnace and may contribute to the formation of NOx. Water jackets are subject to corrosion when the furnace atmosphere contains condensable vapors.
Thermocouples and bimetallic elements when used to monitor the combustion process within the fuel nozzles, suffer from the disadvantages of providing only localized point measurements and generally slow reaction times (typically 2 to 3 minutes), which can lead to problems and possible failure of the fuel nozzle before detection. Another disadvantage of these sensors is that, since they only detect heat, they are unable to distinguish between heat generated by the flame of a flashback condition and the heat radiated by the normal combustion process of the gas turbine combustion system.
Additionally, control of the combustion process necessitates ongoing monitoring of the chemical compositions of the fuel, oxidant, and the products of combustion. Due to the extreme environmental conditions a number of problems must be addressed as part of a combustion control system.
Placement of an in-situ oxygen sensor at the burner exhaust can provide a control solution for overall combustion ratio control. However, typical oxygen sensors, such as zirconia-base sensors that are commercially available have limited lifetime and need to be replaced frequently. One difficulty met when using these sensors is a tendency to plug, especially when the exhaust gases contain volatile species or particulate. Further, when more than one burner is utilized, a drawback of global combustion control is that it is not possible to know whether each individual burner is properly adjusted or not. This technique also has long response times due to the residence times of the burner gases in the combustion chamber, which can exceed 30 seconds.
Continuous monitoring carbon monoxide of the flue gas, for example in so-called post combustion control of a burner assembly, provides another means of controlling the combustion. This involves the use of a sophisticated exhaust gas sampling system, with separation of the particulate matter and of the water vapor. Although very efficient, these techniques are not always economically justified. Also, the light emissions observed from the flame are one of the most useful systems for providing information on the chemical, as well as physical processes, as noted hereinabove, that take place in the combustion process. For example, Cusack et al., U.S. Pat. No. 6,071,114 uses a combination of ultraviolet, visible and infrared measurements to characterized the flame to determine relative levels of some chemical constituents. While monitoring the flame light emission can be easily performed in well controlled environments typically found in laboratories, implementing flame light emission monitoring on industrial burners used in large combustion units is quite difficult in practice, resulting in a number of problems. First, clear optical access is necessary which requires positioning of a viewing port in a strategic location with respect to the flame for collecting the flame light emission. Second, the environment is difficult because of excessive heat being produced by the burner. Typically the high temperature-operating environment of the burners necessitates the need for water or gas cooled probes for use either in or near the burner. Finally, the environment may be dusty which is not favorable for the use of optical equipment except with special precautions, such as gas purging over the optical components.
Control of the combustion process at the burner can be performed by metering the flows of fuel and oxidant, through appropriately regulated valves (electrically or pneumatically driven) that controlled by a programmable controller (PC). The ratio of oxidant to fuel flow is predetermined using the chemical composition of the natural gas and of the oxidant. To be effective, the flow measurements for the fuel and oxidant must be very accurate and readjusted on a regular basis. Typically this situation often leads the operator to use a large excess of air to avoid the formation of CO. Further, typical combustion control strategies do not account for the air intakes that naturally occur in industrial burners that bring in unaccounted quantities of oxidant into the combustion zone, nor does this control scheme account for the variation of the air intakes caused by pressure changes in the burner. Another drawback is that the response time of the feed-forward regulation loop is generally slow, and cannot account for cyclic variatons of oxidant supply pressure and composition that occur when the oxidant is not pure oxygen. Other drawbacks of combustion control strategy result from variations due to fuel composition and pressure.
Other combustion control systems use acoustic control of flames. Most of these systems were developed for small combustion chambers in order to avoid extinction of flames, and are triggered by instabilities of flames.
While currently available systems have been able to achieve some degree of control over the combustion in a burner, there is a need for a fast response time monitoring and control system that is durable, and yet requires minimal modification of the burner assembly and the operating parameters of the burner in order to avoid the previously described problems.
Flame Ionization
Volumes of literature describe investigations of electrical conductivity through gases. The electrical properties of flames and the mechanisms for the formation of ions in flames have been studied extensively. The flame ionization detector (FID) commonly used in gas chromatography uses the electrical properties of flames to determine very low concentration of hydrocarbons. Many investigations using hydrocarbon flames suggest that a large portion of the ionization result from “chemical ionization” in the flame front. Consequently, the reaction most often cited for providing the FID response results from the chemical ionization of CHO*:CH+O→CHO*→CHO+e−
Although the mechanism for providing the response is still debated, the FID is considered a carbon counting device. The FID response is proportional to the number of carbon atoms or the concentration of hydrocarbons in the sample. Cheng et al., The Fast-Response Flame Ionization Detector, Prog. Energy Combustion Science, vol. 24, 1998, pp. 89-124, described the equation for the current measured in the FID asi=r[CnHm]Q,
where r is the charge per mole of hydrocarbon, [CnHm] is the molar concentration of the hydrocarbons, and Q is the volumetric flow rate. The linearity of the FID measurements depends on the consistency of charge collection. This is accomplished mainly by providing consistent inlet bulk flow velocity, providing a constant electric field across the flame, and using a hydrogen flame to ignite the inlet sample and maintain a consistent flame anchor.
Other investgations have shown the feasibility of using flame ionization of monitoring and control of internal combustion (IC) engines. Eriksson et al., Ionization Current Interpretation for Ignition Control in Internal Combustion Engines, L. Eriksson, and L. Nielsen, Control Engineering Practice, Vol. 5 (8), 1997, pp. 1107-1113, demonstrated the feasibility of using in cylinder ionization-current measurements to control IC engine spark advance. Watterfall et al., “Visualizing Combustion Using Electrical Impedance Tomography, Chemical Engineering Science, vol. 52, Issue 13, Jul. 1997, pp. 2129-2138, demonstrated using impedance tomography to visualize combustion in an IC engine. The results of Waterfall show a linear variation of capacitance with the operating air-to-fuel ratio. The main similarity is the use of a direct-current (DC) electric field to yield a current measurement that relates to the flame parameters.
Safety of operation is an essential characteristic expected from all industrial combustion systems. Automated control of the presence of the flame in the combustion can be used to stop the flow of oxidant when the fuel flow is suddenly interrupted.